Although the human eye can sense only those images which are formed by visible light, similar images are also created by invisible forms of electromagnetic radiation such as x-ray, ultraviolet, and infrared radiation. In fact, many objects spontaneously emit various forms of radiation which, like reflected visible light, produce vivid images. For example, the nighttime sky includes images formed by visible light from the sun which is reflected off of the moon and visible light which is spontaneously emitted by all of the other stars. The sun and the stars also emit invisible radiation, including infrared or thermal radiation, which creates images that the human eye is incapable of sensing. In general, all warm objects emit some form of infrared radiation that produces a thermal image.
Devices which sense these thermal images have proven to be quite useful in scientific, commercial, and military applications ranging from non-destructive testing and pollution monitoring to target acquisition. For many of these applications, the images are observed from remote positions during what are generally referred to as "heat surveys." A typical aerial heat survey is illustrated in FIG. 1 where reflected and emitted infrared radiation from the ground are collected by an imaging system on board aircraft 2.
At any one time during the survey in FIG. 1, the static field of view for the imaging system in aircraft 2 will be limited to just one small area on the ground depicted by the ground resolution element 4. However, the overall field of view for the imaging system can be significantly increased by rolling the aircraft 2 from side to side in order to scan a much wider area 6. Since the aircraft is also moving forward while the imaging system is scanning from side to side, a total dynamic field of view 8 can be observed with each pass of the aircraft 2.
Although FIG. 1 has been described above in terms of passive infrared imaging from an aerial vantage point, other forms of invisible and visible radiation might also be detected from a variety of positions. The imaging system might also be "steered" in order to continuously receive radiation from just one ground resolution element 4 as the aircraft 2 moves over that element. Furthermore, fast steering may be used to correct for vibrational distortions in the image, such as "line of sight" errors, which are discussed in more detail below.
Imaging systems with both steering and scanning capabilities are sometimes referred to as "steering/scanning" systems. The imaging system could also be arranged as "forward-looking" so that it receives radiation from sources other than those on the ground. The aircraft 2 might even be fitted with an active source of radiation, such as spotlight or a high intensity laser, that could be used to illuminate specific objects during a survey.
FIG. 2 illustrates a typical electro-optical imaging system for use in conducting the aerial survey illustrated in FIG. 1. In practice, it has been found useful to provide such imaging systems with a small, lightweight mirror assembly 10 which can be rolled from sided to side much more easily than rolling the entire aircraft 2. Radiation from outside the aircraft 2 is reflected off of the mirror assembly 10 through a series of focusing elements 12 to an optical sensor 14. Electrical signals from the optical sensor 14 are then processed by an electronics package 16 before being displayed in a visual format at a monitor 18.
Vibrations in the aircraft which are transmitted to the mirror assembly 10 and focussing elements 12 may cause "line of sight" errors in the resulting image. These vibrational movements will distort the image which is received by optical sensor 14 in much the same way that a photographer's shaky hand on a camera lens will distort the resulting image in a photograph. In either case, the image distortion may be prevented by either stabilizing the imaging system or "steering" it very quickly in order to compensate for the vibrations. However, since it is generally much more difficult to eliminate vibrations in a moving aircraft than it is to steady a stationary imaging system (such as a camera) on the ground, fast steering mirrors are usually associated only with moving imaging systems such as those used for aerial surveys.
As illustrated in FIG. 2, the mirror assembly 10 may be positioned to rotate about two perpendicular axes. These rotations are sometimes referred to as "pitch" and "yaw." In order to accurately resolve the reflected images on sensor 14, however, the mirror assembly 10 must not be allowed to move or rotate along the other axes, and it must be held very still once it is in position. Consequently, the support structure for the mirror must be rigid enough to minimize any deflections due to vibration (which might cause line of sight errors) along the stationary axes and yet still flexible enough along two rotational axes to allow the mirror to be quickly and accurately positioned. Furthermore, the final position of the mirror must be continuously compensated for the effects of vibration.
The effects of vibrations on a structure are often described in terms of natural frequencies, or natural modes of vibration. When any structure is subjected to an impulse, i.e., a sudden force for a very short duration, that structure will vibrate at one or more natural frequencies, or modes, defined by physical characteristics of the structure such as its shape, mass, and flexibility. For example, when two fine crystal glasses are clinked together during a toast (i.e., when they are subjected to an impulse), the edges of the glasses will move back and forth thousands of times, or cycles, per second. Depending on the quality of the crystal, the two glasses will vibrate with at least one frequency in the audible range of 20,000 to 20,000,000 Hertz (cycles per second) to produce an aesthetically pleasing and audible "ring." At the same time, the glasses may also ring at other frequencies which are outside of this acoustic bandwidth defining the normal range of hearing for the human ear. The loudness of the ring will be determined by the amplitude of the vibrations, i.e., the distance that the edges of the glass are deflected during each complete cycle of vibration. Obviously, if the glasses are clinked together too loudly, then they may deflect so far that they actually crack or shatter.
Structural vibrations may also be caused by mechanical energy in other forms besides impulses. For example, each of the glass structures described above would also vibrate if they were subjected to an oscillating source of energy, such as a loudspeaker. When placed in front of the loudspeaker, the glass will tend to vibrate at the same frequency, or frequencies, that are being emitted by the loudspeaker. The glass will vibrate the most (i.e., it will deflect the farthest) when it is driven by the loudspeaker at a natural frequency of the glass structure. In fact, when driven with sufficient energy at a natural frequency, the glass may deflect so far during each cycle that it breaks. Several operatic divas have been known to exhibit the power of their voices by dramatically shattering fine crystal glassware in this manner.
In a similar manner, during normal operation, the mirror assembly 10 will be excited by vibrations, including both impulses and oscillations, from the body of the aircraft 2. The vibrations in the aircraft 2 may stem from a variety of sources such as the rotation of the engines, air flow over the wings, and movement of the payload. As long as these vibrations do not occur at a frequency near the natural frequency of the mirror support structure, then the reflective surface of the mirror in mirror assembly 10 will remain relatively steady. Even small vibrations near the natural frequency of the mirror support structure, however, may cause the support structure to deflect so wildly as to distort the image reflected to sensor 14.
The mirror support structure must thus be rigidly fixed to the body of the aircraft 2 and yet also flexible enough to rotate around two perpendicular axes in order to provide an adequate field of view for the imaging system. Rotational flexibility is particularly important during scanning operations where the mirror must be continuously rotated back and forth. For steering operations, on the other hand, and especially for fast steering operation to correct for line of sight errors, the mirror must be rotated into position and then held steady. It is therefore important during steering for the structure to be stiff enough to remain unaffected by vibrations and impulses once the mirror assembly is moved into position. During both steering and scanning operations, the position of the mirror must also be continuously compensated, or controlled, for the effects of vibration.